We demonstrate that the length of carbon nanotubes (CNTs) can be determined simply and accurately from extensional viscosity measurements of semidilute CNT solutions. The method is based on measuring the extensional viscosity of CNT solutions in chlorosulfonic acid with a customized capillary thinning rheometer and determining CNT aspect ratio from the theoretical relation between extensional viscosity and aspect ratio in semidilute solutions of rigid rods. We measure CNT diameter d by transmission electron microscopy (TEM) and arrive at CNT length L. By studying samples grown by different methods, we show that the method works well for CNT lengths ranging from 0.4 to at least 20 μm, a wider range than for previous techniques. Moreover, we measure the isotropic-to-nematic transition concentration (i.e., isotropic cloud point) φ iso of CNT solutions and show that this transition follows Onsager-like scaling φ iso ∼ d/L. We characterize the length distributions of CNT samples by combining the measurements of extensional viscosity and transition concentration and show that the resulting length distributions closely match distributions obtained by cryo-TEM measurements. Interestingly, CNTs appear to have relatively low polydispersity compared to polymers and high polydispersity compared to colloidal particles.

Confined smectic A liquid crystals (SmA LCs) form topological defects called focal conic domains (FCDs) [1] that focus light as gradient-index lenses [2][3][4][5]. Here, we exploit surface curvature to self-assemble FCDs in a single step into a hierarchical structure [6,7] (coined "flower pattern") molded by the fluid interface that is pinned at the top of a micropillar. The structure resembles the compound eyes of some invertebrates, which consist of hundreds of microlenses on a curved interface, able to focus and construct images in three dimensions (3D) [8]. Here we demonstrate that these flowers are indeed "compound eyes" with important features which have not been demonstrated previously in the literature. The eccentric FCDs gradually change in size with radial distance from the edge of the micropillar, resulting in a variable microlens focal length that ranges from a few microns to a few tens of microns within a single "flower". We show that the microlenses can construct a composite 3D image from different depth of field (DOF). Moreover, the smectic "'compound eye" can be reconfigured by heating and cooling at the LC phase transition temperature; its field of view (FOV) can be manipulated by tuning the curvature of the LC interface, and the lenses are sensitive to light polarization.Insects' eyes are comprised of hundreds of microlenses (ommatidia) arranged on a curved surface [8]. Despite having modest resolution in comparison to single aperture lenses (like the human eyes), the compound eye offers attractive optical properties, including exceptionally wide FOV, fast motion detection, and polarization sensitivity. Artifical compound eyes have been created with angular sensitivity [9] and with a hemispheric FOV and near infinite DOF [10]. Typically, multiple top-down fabrication steps are required, including photolithography, replica molding, or complex micromachining processes [9][10][11]. For practical applications with wide FOV, lenses which self-align are highly desirable. * These authors contributed equally to the work † kstebe@seas.

The ability to dictate the motion of microscopic objects is an important challenge in fields ranging from materials science to biology. Field-directed assembly drives microparticles along paths defined by energy gradients. Nematic liquid crystals, consisting of rod-like molecules, provide new opportunities in this domain. Deviations of nematic liquid crystal molecules from uniform orientation cost elastic energy, and such deviations can be molded by bounding vessel shape. Here, by placing a wavy wall in a nematic liquid crystal, we impose alternating splay and bend distortions, and define a smoothly varying elastic energy field. A microparticle in this field displays a rich set of behaviors, as this system has multiple stable states, repulsive and attractive loci, and interaction strengths that can be tuned to allow reconfigurable states. Microparticles can transition between defect configurations, move along distinct paths, and select sites for preferred docking. Such tailored landscapes have promise in reconfigurable systems and in microrobotics applications.

The ability to control the movement and assembly of particles in liquid crystals is not only an important route to design functional materials, but also sheds light on the mechanisms of colloidal interactions. In this study we place micron-sized particles with "Saturn ring" defects near a wall with hills and dales that impose perpendicular (homeotropic) molecular anchoring. The strong splay distortion at the wall interacts with the distortion around the particles in the near field and favors their migration towards the dales via the so-called "lock-and-key" mechanism. We demonstrate experimentally that the lock-and-key mechanism can robustly localize a particle at specific topographical features. We observe the complex trajectories traced by the particles as they dock on the dales, estimate the binding energy, and explore a range of parameters that favor or disfavor the docking event, thus exploiting the capabilities of our experimental system. We extend the study to colloids with homeotropic anchoring but with an associated point defect instead of a Saturn ring and show that they find a different preferred location, i.e. we can place otherwise identical particles at well defined sites according to their topological defect structure. Finally, for deep enough wells, confinement drives topological transitions of Saturn rings to dipoles. This ability to tailor wall geometry to guide colloids to well defined sites within nematic liquid crystals represents an important new tool in directed assembly.Fabrication through directed self-assembly provides a major simplification in processing, creating, and controlling supramolecular materials. This approach has revolutionized polymer science and is a modality that pervades biology.[1, 2] Soft materials are ideal systems to create long-range interactions by molding the associated free energy fields within which inclusions move, interact and assemble. For 1 arXiv:1602.05639v4 [cond-mat.soft]

Coaxial cables for data transmission are ubiquitous in telecommunications, aerospace, automotive, and robotics industries. Yet, the metals used to make commercial cables are unsuitably heavy and stiff. These undesirable traits are particularly problematic in aerospace applications, where weight is at a premium and flexibility is necessary to conform with the distributed layout of electronic components in satellites and aircraft. The cable outer conductor (OC) is usually the heaviest component of modern data cables; therefore, exchanging the conventional metallic OC for lower weight materials with comparable transmission characteristics is highly desirable. Carbon nanotubes (CNTs) have recently been proposed to replace the metal components in coaxial cables; however, signal attenuation was too high in prototypes produced so far. Here, we fabricate the OC of coaxial data cables by directly coating a solution of CNTs in chlorosulfonic acid (CSA) onto the cable inner dielectric. This coating has an electrical conductivity that is approximately 2 orders of magnitude greater than the best CNT OC reported in the literature to date. This high conductivity makes CNT coaxial cables an attractive alternative to commercial cables with a metal (tin-coated copper) OC, providing comparable cable attenuation and mechanical durability with a 97% lower component mass.

Anti-icing surfaces/interfaces are of considerable importance in various engineering fields under natural freezing environment. Although superhydrophobic self-cleaning surfaces show good anti-icing potentials, promotion of these surfaces in engineering applications seems to enter a "bottleneck" stage. One of the key issues is the intrinsic relationship between superhydrophobicity and icephobicity is unclear, and the dynamic action mechanism of "air cushion" (a key internal factor for superhydrophobicity) on icing suppression was largely ignored. Here we report that icing inhibition (i.e., icing-delay) of self-cleaning surfaces is mainly ascribed to air cushion and its convection. We experimentally found air cushion on the porous self-cleaning coating under vacuum environments and on the water/ice-coating interface at low temperatures. The icing-delay performances of porous self-cleaning surfaces compared with bare substrate, up to 10-40 min under 0 to ∼-4 °C environments close to freezing rain, have been accurately real-time recorded by a novel synergy method including high-speed photography and strain sensing voltage. Based on the experimental results, we innovatively propose a physical model of "air cushion convection inhibiting icing", which envisages both the static action of trapped air pocket without air flow and dynamic action of air cushion convection. Gibbs free energy of water droplets increased with the entropy of air derived from heat and mass transfer between warmer air underneath water droplets and colder surrounding air, resulting in remarkable ice nucleation delay. Only when air cushion convection disappears can ice nucleation be triggered on suitable Gibbs free energy conditions. The fundamental understanding of air cushion on anti-icing is an important step toward designing optimal anti-icing surfaces for practical engineering application.

Focal conic domains (FCDs) form in smectic-A liquid crystal films with hybrid anchoring conditions with eccentricity and size distribution that depend strongly on interface curvature. Assemblies of FCDs can be exploited in settings ranging from optics to material assembly. Here, using micropost arrays with different shapes and arrangement, we assemble arrays of smectic flower patterns, revealing their internal structure as well as defect size, location, and distribution as a function of interface curvature, by imposing positive, negative, or zero Gaussian curvature at the free surface. We characterize these structures, relating free surface topography, substrate anchoring strength, and FCD distribution. Whereas the largest FCDs are located in the thickest regions of the films, the distribution of sizes is not trivially related to height, due to Apollonian tiling. Finally, we mold FCDs around microposts of complex shape and find that FCD arrangements are perturbed near the posts, but are qualitatively similar far from the posts where the details of the confining walls and associated curvature fields decay. This ability to mold FCD defects into a variety of hierarchical assemblies by manipulating the interface curvature paves the way to create new optical devices, such as compound eyes, via a directed assembly scheme.

We study colloids suspended in nematic liquid crystal in grooves with homeotropic anchoring. We observe "eyelashes", topological dipole chains that follow the local, curved director field. These beget wires that connect the groove corners to topographical features on the cell lid to yield oriented, curvilinear colloidal wires spanning the cell, formed in a non-singular director field. As the groove aspect ratio changes, we find different ground states and corroborate our observation with numerics. Our results rely upon on the scale of topographical features, the sharpness of edges, and the colloidsourced distortions; all these elements can be exploited to guide the formation of reconfigurable structures in nematics.